Filament trimmers are well known products that are used to trim grass, weeds, or similar vegetation. They comprise an elongated handle assembly having a hand grip at the upper end and a trimmer head rotatably mounted at the lower end. The trimmer head includes a spool that carries a coiled supply of filament line with a free end of this line extending radially outwardly from the head. When a motor carried on the trimmer is activated, the head is rotated to spin the free end of the line about the rotational axis of a drive shaft to allow the free end of the line to sever grass, weeds and the like.
Despite the relatively well developed nature of these products, the filament trimmers existing today have various flaws. For example, the spools which carry the coiled supply of the line are typically molded from plastic with the spools having a cylindrical central core on which the line is wound. However, the filament line is sensitive to environmental conditions, and particularly to the humidity of the ambient air, and can expand and contract with changes in these conditions. When the line contracts on the core, it can in some instances impose enough of a compressive force on the core to warp or damage the spool. This can cause the spool to no longer operate properly or at all, which is a disadvantage.
The filament line carried on the spool typically extends outwardly from the spool through an exit aperture in a peripheral sidewall of the head. It can be difficult to thread the line through this aperture during installation of the spool due to the tight spaces found inside the head. Some trimmers have attempted to solve this problem by providing an exit aperture that can be opened up to allow the line to be laid into the aperture when various parts comprising the head are disassembled, but is otherwise closed when the head is in its assembled state. This approach is shown in U.S. Pat. No. 4,672,798 to Ota. However, the exit aperture in this approach is typically formed as an elongated U-shaped slot that is quite long and which does not constrain the line against vertical movement as well as the generally circular exit aperture which it replaces. This allows the line to move too much inside the aperture, leading to melting of the line at the aperture due to friction or to breaking of the line from abrasion.
Most filament trimmer heads have an indexing system for feeding out additional line to replenish the cutting length whenever the line has become shortened due to wear or breakage. Many of these systems operate by normally locking the spool relative to the head. However, when more line is required, the spool is unlocked to allow the spool to rotate relative to the head, thereby feeding out more line from the spool and thus extending the cutting length. In one indexing system of this type, the unlocking of the spool is initiated by bumping the trimmer head, and specifically a downwardly protruding ground engagement member on the head, against the ground. Such a ground bump indexing system is shown in U.S. Pat. No. 4,524,515 to Oberg, which is assigned to The Toro Company, the assignee of this application.
Ground bump indexing heads have an opening provided therein to allow the ground engagement member to protrude out of the head. This opening, which normally faces the ground, provides an entry point into the interior of the head for dirt, sand, grit and the like. If the trimmer is operated in dirty environments where a lot of particulate material is present, the inside of the trimmer head can become so contaminated that the indexing system either no longer works reliably or can be damaged. This requires the trimmer head to be disassembled for cleaning or potentially requires some of the components of the indexing system to be repaired or replaced.
In addition, most line indexing systems, including ground bump indexing systems, have various sets of teeth that engage with one another to lock the spool relative to the head. Some of these teeth are provided on the head and some on the spool. The most economical way of providing teeth on the spool is to mold such teeth integrally into the spool. This is most often done by molding such teeth into the upper face of one of the spool flanges or into the peripheral edge of one or more of the spool flanges, as shown in U.S. Pat. No. 4,349,962 to Itagaki et al.
However, if the spool flanges warp even slightly and become distorted from their normal horizontal orientation, the teeth carried on the spool flanges may not reliably engage the teeth provided on the head. This leads to imperfect and unreliable operation of the indexing system. In addition, outwardly extending teeth on the peripheral edge of the spool flange unnecessarily increases the diameter of the spool, and thus the diameter of the head, and potentially serve as obstructions that can catch the line extending out from the spool.
Finally, most filament trimmer heads are mounted onto the motor shaft by being threaded onto that shaft. If the threads provided in the head are integrally molded into the head, the cycle times for molding the head are considerably reduced, leading to a more expensive head. If the threads are provided in a separate metallic insert, the cost of the insert, and the cost to mold it in place into the head, must also be borne. Moreover, in an electric trimmer, this threaded metallic insert has to be molded into a separate plastic part to electrically insulate it. Thus, these two prior art methods of providing attachment threads in filament trimmer heads for securing the head to the motor shaft are relatively expensive. A less expensive way of attaching the trimmer head to the motor shaft is desirable.